Parent-Child Interaction Therapy (PCIT) is an empirically supported treatment for disruptive behavior disorders (DBD) in young children. Approximately 70% of children with a DBD in PCIT studies have a comorbid diagnosis of ADHD. Yet PCIT has not been examined as a treatment for ADHD-related behaviors per se. The objective of this project is to examine a prescriptive model of psychosocial treatment for ADHD in preschoolers. The characteristic of prescriptive interest is the presence or absence of comorbid DBD [Oppositional Defiant Disorder (ODD) or Conduct Disorder (CD)]. The treatments to be compared are Individual PCIT and Group PCIT. A prescriptive model of treatment is proposed to test the hypothesis that families of young children with ADHD who have comorbid ODD or CD will require the more intensive, individualized treatment format (namely, Individual PCIT) to change child behavior, whereas children with ADHD-only will benefit equally from either treatment format. This project has three specific aims: (a) To determine whether children with ADHD-only will differ from children with ADHD + ODD/CD on treatment outcome measures at post-treatment and follow-up;(b) To conduct an economic evaluation of Group PCIT and Individual PCIT for treating ADHD-related behaviors;and (c) To identify determinants of immediate and longer-term treatment response to Individual and Group PCIT formats. Multiple informants and methods of measurement will be used to assess parenting skills, parent psychopathology, and child ADHD-related behaviors at home and school. Participants will be 128 families with a young child (4 to 6 years) with ADHD. Families will be randomly assigned to one of two groups: (a) Individual PCIT, or (b) Group PCIT, stratified by comorbid diagnostic status. Half (64) the children (32 with ADHD-only;32 with ADHD + ODD/CD) will receive Individual PCIT. The other 64 children (32 with ADHD only;32 with ADHD + ODD/CD) will receive Group PCIT. Assessments will be conducted at pre- and post-treatment and at 1- and 2-year follow-up points. We expect significant improvement for all children on outcome measures at post-treatment and follow-up assessments. We expect children with ADHD + ODD/CD to show better outcomes after Individual PCIT than Group PCIT on all measures at post-treatment and follow-up. We expect equivalence in primary outcome measures between Individual PCIT and Group PCIT for children with ADHD-only, but we expect the costs of Group PCIT to be substantially lower, providing additional evidence of the utility of prescriptive treatments. We will explore additional moderators of outcome and test models of mechanisms for change by examining intervention- targeted mediators, including parenting skills and parent psychopathology.